U.S. patent application number 13/683320 was filed with the patent office on 2013-05-30 for stimulated raman scattering measurement apparatus.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. The applicant listed for this patent is CANON KABUSHIKI KAISHA. Invention is credited to Kazuyoshi ITOH, Yasuyuki OZEKI, Wataru UMEMURA.
Application Number | 20130135615 13/683320 |
Document ID | / |
Family ID | 47357921 |
Filed Date | 2013-05-30 |
United States Patent
Application |
20130135615 |
Kind Code |
A1 |
OZEKI; Yasuyuki ; et
al. |
May 30, 2013 |
STIMULATED RAMAN SCATTERING MEASUREMENT APPARATUS
Abstract
The measurement apparatus combines first and second lights from
first and second light generators to focus the combined light to a
sample, and detect the first or second light intensity-modulated by
stimulated Raman scattering. The first light generator includes a
light dispersion element separating the light from a light
introducing optical system in different directions according to
light frequencies, and drives at least one of the light dispersion
element and part of the light introducing optical system so as to
change an incident angle of the light to the light dispersion
element to extract part of the separated light, thereby making a
light frequency of the first light variable. The second light
generator produces a plurality of the second lights having mutually
different light frequencies. The apparatus measures a Raman
spectrum by changing the light frequency of the first light.
Inventors: |
OZEKI; Yasuyuki; (Minoh-shi,
JP) ; ITOH; Kazuyoshi; (Kawanishi-shi, JP) ;
UMEMURA; Wataru; (Suita-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CANON KABUSHIKI KAISHA; |
Tokyo |
|
JP |
|
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
47357921 |
Appl. No.: |
13/683320 |
Filed: |
November 21, 2012 |
Current U.S.
Class: |
356/301 |
Current CPC
Class: |
H01S 3/1618 20130101;
G01J 3/4338 20130101; H01S 3/0078 20130101; H01S 3/1611 20130101;
H01S 3/0092 20130101; H01S 3/06754 20130101; G01J 3/10 20130101;
G01N 21/65 20130101; G01N 2021/655 20130101; H01S 3/2391 20130101;
H01S 3/1307 20130101; G01J 3/44 20130101 |
Class at
Publication: |
356/301 |
International
Class: |
G01N 21/65 20060101
G01N021/65 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 25, 2011 |
JP |
2011-257894 |
Claims
1. A stimulated Raman scattering measurement apparatus comprising:
a first light generator configured to produce a first light; a
second light generator configured to produce a second light having
a light frequency different from that of the first light; an
optical system configured to combine the first light with the
second light to focus the combined first and second lights to a
sample; and a detector configured to detect the first or second
light intensity-modulated by stimulated Raman scattering caused by
the focusing of the combined first and second lights to the sample,
wherein the first light generator includes a light introducing
optical system and a light dispersion element, the light
introducing optical system introducing light from a first light
source to the light dispersion element, and the light dispersion
element being operable to separate the light introduced thereto in
different directions according to light frequencies, wherein the
first light generator is configured to drive at least one of the
light dispersion element and an optical element included in the
light introducing optical system so as to change an incident angle
of the light from the first light source to the light dispersion
element to extract part of the separated light in the mutually
different directions, thereby being configured to make a light
frequency of the first light variable, wherein the second light
generator is configured to produce a plurality of the second lights
having mutually different light frequencies by using light from at
least one second light source, and wherein the apparatus is
configured to measure a Raman spectrum by changing the light
frequency of the first light.
2. A stimulated Raman scattering measurement apparatus according to
claim 1, wherein the light dispersion element is an optical element
having a diffraction grating.
3. A stimulated Raman scattering measurement apparatus according to
claim 1, wherein the second light generator includes a highly
nonlinear fiber operable to expand a light frequency band of the
light from the second light source and harmonic generation elements
operable to respectively generate harmonics of light components,
which have mutually different frequencies, of the light from the
highly nonlinear fiber, and wherein the second light generator is
configured to produce the second lights by using the harmonics.
4. A stimulated Raman scattering measurement apparatus according to
claim 1, wherein, when difference among the light frequencies of
the second lights is represented by W.sub.p, a variable width of
the light frequency of the first light is equal to or more than
W.sub.P.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a stimulated Raman
scattering measurement apparatus performing molecular vibration
imaging by utilizing stimulated Raman scattering, the apparatus
being particularly suitable for a microscope, an endoscope and the
like.
[0003] 2. Description of the Related Art
[0004] A stimulated Raman scattering (SRS) measurement apparatus
that is one of measurement apparatuses utilizing a Raman scattering
principle has been proposed in "Principle confirmation of
stimulated Raman scattering microscopy" (Optics&Photonics Japan
2008, 5pC12) by Fumihiro Dake, Yasuyoki Ozeki and Kazuyoshi Ito and
"Label-free biomedical imaging with high sensitivity by stimulated
Raman scattering microscopy" (SCIENCE VOL322 19 Dec. 2008 pp.
1857-1861) by Chiristian W. Freudiger, Wei Min, Brian G. Saar,
Sijia Lu, Gary R. Holtom, Chengwei He, Jason C. Tsai, Jing X. Kang
and X. Sunney Xie. The principle of the SRS detection apparatus is
as follows.
[0005] When two light pulses whose light frequencies are mutually
different are focused to a sample, a coincidence of difference
between the light frequencies of the two light pulses with a
molecular vibration frequency of the sample causes a phenomenon of
stimulated Raman scattering at a light-focused point. The
stimulated Raman scattering decreases intensity of one of the two
light pulses having a higher light frequency, and increases
intensity of the other one having a lower light frequency.
Detection of such intensity change enables molecular vibration
imaging in which vibration information of molecules of the sample
is reflected. In this detection, performing intensity modulation on
one of the two light pulses or providing to the one light pulse a
repetition frequency that is an integral multiple of that of the
other light pulse modulates the intensity change caused by the
stimulated Raman scattering. Therefore, detecting the modulation of
the intensity change of the other light pulse (on which the
intensity modulation is not performed) by a lock-in amplifier
enables detection of the stimulated Raman scattering with high
sensitivity.
[0006] For such a stimulated Raman scattering measurement
apparatus, it is expected that its discrimination ability for the
sample may further improve, not by detecting the molecular
vibration only at a specific light frequency, but by detecting a
molecular vibration spectrum (hereinafter referred to as "a Raman
spectrum") in a wide vibration frequency range. Japanese patent
Laid-Open No. 2010-48805 and "Label-Free and Real-Time Monitoring
of Biomass Processing with Stimulated Raman Scattering Microscopy
(Supporting Information)" (published by Harvard University as a
research paper in 2010) disclose stimulated Raman scattering
measurement apparatuses capable of scanning the light frequency of
one of the above-mentioned two light pulses by using one or more
optical parametric oscillators (hereinafter referred to as an
"OPO") having an oscillation wavelength variable function.
[0007] The OPO can scan a comparatively wide light frequency range
by changing not only a tilt of a crystal that outputs the above one
light pulse, but also temperature thereof. Moreover, "Fiber-format
stimulated-Raman-scattering microscopy from a single laser
oscillator" (published by Politecnico di Milano as a research paper
in 2010) discloses a stimulated Raman scattering measurement
apparatus capable of scanning the light frequency of one of the
above two light pulses by changing temperature of a periodically
poled element, such as periodically poled lithium niobate (PPLN),
from which the one light pulse exits.
[0008] However, the stimulated Raman scattering measurement
apparatuses disclosed in Japanese patent Laid-Open No. 2010-48805,
"Label-Free and Real-Time Monitoring of Biomass Processing with
Stimulated Raman Scattering Microscopy (Supporting Information)"
and "Fiber-format stimulated-Raman-scattering microscopy from a
single laser oscillator" gradually perform scanning of the light
frequency by changing the temperature of the crystal in the OPO or
of the periodically poled element, so that it takes a long time to
acquire the Raman spectrum in a wide light frequency range. As
methods for performing the scanning in a wide light frequency range
in a short time, there are known a method of filtering wideband
laser light by an acoustooptic tunable filter (AOTF) and a method
of providing a chirp to two laser light pulses whose light
frequencies are mutually different and controlling a time
difference between the pulses to change a molecular vibration
frequency. However, these methods cannot provide sufficient
spectral resolution in the stimulated Raman scattering measurement
apparatus.
SUMMARY OF THE INVENTION
[0009] The present invention provides a stimulated Raman scattering
measurement apparatus capable of acquiring the Raman spectrum with
high resolution and at high speed.
[0010] The present invention provides as one aspect thereof a
stimulated Raman scattering measurement apparatus including a first
light generator configured to produce a first light, a second light
generator configured to produce a second light having a light
frequency different from that of the first light, an optical system
configured to combine the first light with the second light to
focus the combined first and second lights to a sample, and a
detector configured to detect the first or second light
intensity-modulated by stimulated Raman scattering caused by the
focusing of the combined first and second lights to the sample. The
first light generator includes a light introducing optical system
and a light dispersion element, the light introducing optical
system introducing light from a first light source to the light
dispersion element, and the light dispersion element being operable
to separate the light introduced thereto in different directions
according to light frequencies. The first light generator is
configured to drive at least one of the light dispersion element
and an optical element included in the light introducing optical
system so as to change an incident angle of the light from the
first light source to the light dispersion element to extract part
of the separated light in the mutually different directions,
thereby being configured to make a light frequency of the first
light variable. The second light generator is configured to produce
a plurality of the second lights having mutually different light
frequencies by using light from at least one second light source.
The apparatus is configured to measure a Raman spectrum by changing
the light frequency of the first light.
[0011] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a block diagram showing a configuration of a
stimulated Raman scattering measurement apparatus that is
Embodiment 1 of the present invention.
[0013] FIG. 2 is a block diagram showing a configuration of a
stimulated Raman scattering measurement apparatus that is
Embodiment 2 of the present invention.
[0014] FIG. 3 is a block diagram showing a configuration of a first
pulse light generator used in the stimulated Raman scattering
measurement apparatus of Embodiment 1.
[0015] FIG. 4 is a block diagram showing a configuration of a
modified example of the first pulse light generator used in the
stimulated Raman scattering measurement apparatuses of Embodiments
1 and 2.
[0016] FIG. 5 is a block diagram showing a configuration of another
modified example of the first pulse light generator used in the
stimulated Raman scattering measurement apparatuses of Embodiments
1 and 2.
[0017] FIGS. 6A to 6C show light frequency scanning in the
stimulated Raman scattering measurement apparatus of Embodiment
1.
[0018] FIG. 7 shows a result of the light frequency scanning of the
first pulse light obtained by an experiment using the stimulated
Raman scattering measurement apparatus of Embodiment 1.
[0019] FIG. 8 shows an image obtained by an experiment using the
stimulated Raman scattering measurement apparatus of Embodiment
1.
[0020] FIGS. 9A to 9D show Raman spectra obtained by experiments
using the stimulated Raman scattering measurement apparatus of
Embodiment 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] Exemplary embodiments of the present invention will
hereinafter be described with reference to the accompanying
drawings.
Embodiment 1
[0022] FIG. 1 shows a configuration of a stimulated Raman
scattering (SRS) measurement apparatus that is a first embodiment
(Embodiment 1) of the present invention. The SRS measurement
apparatus 100 of this embodiment can be used as apparatuses, such
as a microscope and an endoscope, for observation, measurement,
diagnosis and other usages.
[0023] The SRS measurement apparatus 100 of this embodiment
includes a first pulse light generator 1 configured to produce a
first pulse light (first light) to be used as a Stokes light and a
second pulse light generator 2 configured to produce a second pulse
light (second light) to be used as a pump light. Moreover, the
measurement apparatus 100 includes a loop filter (LF) 3 and a
two-photon absorption photodiode (TPA-PD) 4, which are provided to
control emission timings of light sources (described later)
provided in the first and second pulse light generators 1 and 2. In
addition, the measurement apparatus 100 includes a mirror group 5,
a combining mirror (combining element) 6, a first optical system 7,
a second optical system 9, a color filter 10 and a light detector
30. A sample 8 that is a measurement target object is installed
between the first optical system 7 and the second optical system
9.
[0024] The first pulse light generator 1 is constituted of a first
light source 11, a wavelength tunable (that is, light frequency
variable) band-pass filter (TBPF) 12 and a Yb doped fiber amplifier
(YDFA) 13. The first light source 11 repeats emission (oscillation)
of a pulse light with a first pulse period, and is constituted of a
mode-locked Yb fiber laser (YbFL) in this embodiment. The pulse
light emitted from the first light source 11 has, for example, a
central light frequency of .omega..sub.S0 (that is, a wavelength of
1030 nm) and a repetition frequency (.nu..sub.S) of 17.5 MHz.
[0025] The wavelength tunable band-pass filter 12 varies a light
frequency .omega..sub.S of the pulse light entering thereto from
the first light source 11 between .omega..sub.S1 and .omega..sub.S2
(.omega..sub.S1<.omega..sub.S2). A configuration of the
wavelength tunable band-pass filter 12 will be described later. The
Yb doped fiber amplifier 13 amplifies the pulse light exiting from
the wavelength tunable band-pass filter 12. The first pulse light
thus amplified by the Yb doped fiber amplifier 13 and having the
light frequency of .omega..sub.S variable between .omega..sub.S1
and .omega..sub.S2 exits from the first pulse light generator
1.
[0026] Description will be made of the configuration of the
wavelength tunable band-pass filter 12 with reference to FIG. 3.
The wavelength tunable band-pass filter 12 is constituted of a
light introducing optical system, a light dispersion element 125
and a fiber collimator 126. The light introducing optical system is
constituted of a half mirror 121, a light deflecting element 122, a
first lens 123 and a second lens 124. The first lens 123 has a
focal length of f1, and the second lens 124 has a focal length of
f2.
[0027] The pulse light from the first light source (YbFL) 11 is
transmitted through the half mirror 121 to enter the light
deflecting element 122 which is movable. The light deflecting
element 122 is constituted of an optical element that is rotatable
or swingable at high speed and is operable to change a direction of
the light exiting therefrom, such as a galvanic mirror, a polygon
mirror, a resonance scanner or a MEMS (Micro-Electro Mechanical
Systems) mirror. A driver 128 includes an actuator to rotate
(swing) the light deflecting element 122 and an electronic circuit
to drive the actuator.
[0028] The pulse light reflected by the light deflecting element
122 is transmitted through the first and second lenses 123 and 124
to be introduced to the light dispersion element 125. The light
deflecting element 122 changes, by its light deflecting action, an
incident angle of the pulse light entering thereto, as shown by
solid lines and dotted lines in FIG. 3.
[0029] The light dispersion element 125 is operable to separate the
entering light (pulse light) in different directions according to
light frequencies. This embodiment (and also Embodiment 2 described
later) uses an element having a diffraction grating as the light
dispersion element 125. Use of a dispersion effect of the
diffraction grating can sufficiently narrow a spectral width of the
first pulse light. As the light dispersion element, other elements
such as a prism may be used.
[0030] In this embodiment, a distance between the light deflecting
element 122 and the first lens 123 and a distance between the first
lens 123 and its posterior focal point position are coincident with
the focal length f1 of the first lens 123. Moreover, a distance
between the second lens 124 and its anterior focal point position
and a distance between the second lens 124 and the light dispersion
element 125 are coincident with the focal length f2 of the second
lens 124. This configuration constitutes a 4f imaging system.
Therefore, regardless of the light deflecting action of the light
deflecting element 122, the pulse light as a collimated light
enters the light dispersion element 125 without change of its
entrance point on the light dispersion element 125. In addition,
even though the light frequency of the pulse light exiting from the
light dispersion element 125 is changed, group delay (group
velocity) thereof is kept constant.
[0031] The pulse light exiting from the light dispersion element
125 (reflected thereby in Littrow reflection) and having the light
frequency .omega..sub.S (from .omega..sub.S1 to .omega..sub.S2)
according to its incident angle to the light dispersion element 125
is again transmitted through the second and first lenses 124 and
123 and the light deflecting element 122 to enter the half mirror
121. Then, the pulse light is reflected by the half mirror 121 to
enter the fiber collimator 126. Thus, part of the light separated
by the light dispersion element 125 in the different directions is
extracted. The pulse light collimated by the fiber collimator 126
exits therefrom toward the Yb doped fiber amplifier 13.
[0032] If the Yb doped fiber amplifier 13 has a problem that the
group delay is varied according to the light frequency of the first
pulse light, compensation therefor can be made by changing the
distance between the second lens 124 and the light dispersion
element 125. Moreover, using the Yb doped fiber amplifier 13 in a
state where its gain is saturated can suppress variation of output
from the Yb doped fiber amplifier caused by scanning of the light
frequency of the first pulse light.
[0033] In FIG. 1, the second pulse light generator 2 includes a
second light source 21, an Er doped fiber amplifier (EDFA) 22, a
highly nonlinear fiber (HNLF) 23 and plural (four in this
embodiment) periodically poled elements 24a to 24d each constituted
of periodically poled lithium niobate (PPLN). The second light
source 21 repeats emission (oscillation) of a pulse light with a
second pulse period, and is constituted of a mode-locked Er fiber
laser (ErFL) in this embodiment. The pulse light emitted from the
second light source 21 has, for example, a central light frequency
of cop (that is, a wavelength of 1550 nm) and a repetition
frequency (2.nu..sub.S) of 35 MHz, which are different from those
of the pulse light emitted from the first light source 11.
[0034] The Er doped fiber amplifier (EDFA) 22 amplifies the pulse
light emitted from the second light source 21 to cause the
amplified pulse light to exit therefrom.
[0035] The highly nonlinear fiber 23 expands a light frequency band
of the pulse light amplified by the Er doped fiber amplifier 22 to
convert the pulse light into a supercontinuum (SC) pulse light. The
periodically poled lithium niobate elements 24a to 24d serve as
harmonic generation elements that respectively generate second
harmonics of plural (four) pulse light components included in the
SC pulse light from highly nonlinear fiber 23 and having mutually
different light frequencies .omega..sub.p1, .omega..sub.p2,
.omega..sub.p3 and .omega..sub.p4. The four second harmonics
generated by the periodically poled lithium niobate elements 24a to
24d exit from the second pulse light generator 2 simultaneously
with one another as four second pulse lights having the mutually
different light frequencies .omega..sub.p1, .omega..sub.p2,
.omega..sub.p3, and .omega..sub.p4. In this embodiment, the light
frequencies have the following relationship:
.omega..sub.p1<.omega..sub.p2<.omega..sub.p3<.omega..sub.p4.
[0036] This embodiment sets the repetition frequency of the first
pulse light to half of the repetition frequency of the second pulse
light. Therefore, each light pulse of the first pulse light is
produced at a timing synchronizing with a timing corresponding to
two light pulses of the second pulse light. Such setting of the
production timing of the first pulse light enables increase
(maximization) of occurrence frequency of the above-mentioned
effect of the stimulated Raman scattering as compared with a case
of setting the repetition frequency of the first pulse light to 1/3
or 1/4 of that of the second pulse light, which makes it possible
to acquire molecular vibration images of the sample 8 with higher
accuracy. However, the repetition frequency of the first pulse
light is not limited to half of that of the second pulse light, and
may be set to 1/2n (n is an integer equal to or more than 2) such
as 1/4. Moreover, the repetition frequency of the first pulse light
only has to be lower than that of the second pulse light. In other
words, it is only necessary to be able to detect the second pulse
light intensity-modulated by the stimulated Raman scattering by
matching timings at which the first and second pulse lights are
focused to the sample 8.
[0037] The two-photon absorption photodiode (TPA-PD)
photoelectrically converts the pulse light emitted from the first
light source 11 and the pulse light emitted from the second light
source 21 and transmitted through the highly nonlinear fiber 23 to
output a voltage signal showing a timing difference between these
pulse lights. The voltage signal showing the timing difference is
input to the first and second light sources 11 and 21 through the
loop filter 3. Internal circuits of the first and second light
sources 11 and 21 control pulse light emission timings thereof so
as to make the voltage signal constant, that is, so as to provide
the above-mentioned synchronized timings.
[0038] Although this embodiment uses, for the first and second
light sources 11 and 12, the mode-locked Yb or Er fiber laser,
other fiber lasers or lasers other than fiber lasers (such as
titanium sapphire laser) may be used.
[0039] Moreover, although this embodiment uses the first pulse
light having a lower repetition frequency than that of the second
pulse light as the Stokes light, and uses the second pulse light as
the pump light, the first pulse light may be used as the pump light
and the second pulse light may be used as the Stokes light.
[0040] The four second pulse lights exiting from the second pulse
light generator 2 are introduced to the combining mirror 6 by the
mirror group 5, which is constituted of plural dichroic mirrors and
plural mirrors, to be concentrically combined with the first pulse
light exiting from the first pulse light generator 1. The combined
pulse light (first and second pulse lights) is focused to the
sample 8 through the first optical system 7.
[0041] When the repetition frequencies of the first and second
pulse light focused to the sample 8 are respectively represented by
.nu..sub.s and 2.nu..sub.S, focusing of both the first pulse light
and the second pulse lights (four second pulse lights) to the
sample 8 and focusing of only the second pulse light are
alternately performed at a time interval of 1/(2.nu..sub.S). At
each time where both the first and second pulse lights are focused
to the sample 8 at a time interval of 1/.nu..sub.S in a state where
difference between the light frequencies of the first and second
pulse lights, that is, .omega..sub.pn-.omega..sub.S (n=1 to 4) is
coincident with a molecular vibration frequency of a measurement
target molecule in the sample 8, the stimulated Raman scattering is
generated. This stimulated Raman scattering causes intensity
modulation of the second pulse light at a frequency of
.nu..sub.S.
[0042] The first pulse light and the four second pulse lights
intensity-modulated by the stimulated Raman scattering exit from
the sample 8 and then are collected by the second optical system 9.
The first and second pulse lights collected by the second optical
system 9 enter the color filter 10. Of these entering pulse lights,
only the four second pulse lights are transmitted through the color
filter 10 to enter the light detector 30.
[0043] The light detector 30 includes three dichroic mirrors 31a to
31c operable to separate the four pulse lights according to their
light frequencies, and photodiodes 32a to 32d operable to
photoelectrically convert the separated four pulse lights into
electric signals (output signals) corresponding to their light
intensities. Furthermore, the light detector 30 includes four
lock-in amplifiers 33a to 33d into which the output signals from
the four photodiodes 11 are respectively input. The four lock-in
amplifiers 33a to 33d synchronously detect the output signals from
the four photodiodes 11 with a lock-in frequency of .nu..sub.S
shown by a frequency reference signal Ref from the first pulse
light generator 1.
[0044] Thereby, only light (intensity-modulated component of the
second pulse light) caused by the stimulated Raman scattering is
detected. Then, scanning an area on the sample 8 to which the first
and second pulse lights are focused makes it possible for a
calculating unit 34 taking in outputs from the lock-in amplifiers
33a to 33d to acquire a molecular vibration image of the
measurement target molecule in the sample 8. In the scanning, the
light frequency .omega..sub.S (from .omega..sub.S1 to
.omega..sub.S2) of the first pulse light is changed (in other
words, scanned) with respect to the respective light frequencies
.omega..sub.p1, .omega..sub.p2, .omega..sub.p3 and .omega..sub.p4
of the four second pulse lights enables acquisition of a molecular
vibration image and a Raman spectrum in a continuous and wide
frequency range.
[0045] FIG. 6A shows that the light frequency .omega..sub.S
(central light frequency .omega..sub.S0) of the first pulse light
exiting from the first pulse light generator 1 is scanned by the
wavelength tunable band-pass filter 12 within a variable range
W.sub.s from .omega..sub.S1 to .omega..sub.S2. FIG. 6B shows the
four second pulse lights exiting from the second pulse light
generator 2 and having the frequencies .omega..sub.p1,
.omega..sub.p2, .omega..sub.p3 and .omega..sub.p4.
[0046] When W.sub.P represents a frequency pitch that is difference
among the frequencies .omega..sub.p1, .omega..sub.p2,
.omega..sub.p3 and .omega..sub.p4 of the four second pulse lights,
it is desirable that the variable range (scanning width) W.sub.S of
the light frequency of the first pulse light be equal to or more
than W.
[0047] As shown in FIG. 6C, scanning of the light frequency of the
first pulse light with respect to the light frequencies of the four
second pulse lights enables detection of the Raman spectrum in a
continuous and wide vibration frequency range W.sub.R from a
vibration frequency of .omega..sub.p1-.omega..sub.S2 to a vibration
frequency of .omega..sub.p4-.omega..sub.S1. FIG. 6C shows a case
where the scanning width W.sub.S of the light frequency of the
first pulse light is coincident with W.sub.P.
[0048] If a target spectrum range (predetermined light frequency
range) for the detection of the Raman spectrum is decided, the
scanning width of the light frequency of the first pulse light and
the number and light frequencies of the second pulse lights will be
selected so as to enable the scanning in this target spectrum
range.
[0049] This embodiment scans the light frequency of the first pulse
light with respect to the respective light frequencies of the
plural second pulse lights simultaneously exiting from the second
pulse light generator 2 by utilizing high speed rotation of the
light deflecting element 122 in the wavelength tunable band-pass
filter 12. Therefore, this embodiment enables high speed scanning,
and thereby makes it possible to acquire the Raman spectrum in a
wide vibration frequency range in a short time.
[0050] Moreover, this embodiment sufficiently narrows the spectral
width of at each scan point light frequency of the first pulse
light by the wavelength tunable band-pass filter 12, so that this
embodiment enables detection of the Raman spectrum with high
resolution.
[0051] FIG. 7 shows an experiment example of the scanning of the
light frequency of the first pulse light using the wavelength
tunable band-pass filter 12 described in this embodiment. This
experiment resulted in 24 nm as an effective scanning width of the
light frequency of the first pulse light, and resulted in 0.3 nm as
the spectral width at each scan point light frequency. This
spectral width is sufficiently narrow for the high resolution
detection of the Raman spectrum.
[0052] FIG. 8 shows a Raman spectrum of polystyrene (beads each
having a diameter of 10 .mu.m) and a Raman spectrum of PMMA (beads
each having a diameter of 4 .mu.m), which were detected in
experiments using a fast wavelength tunable band-pass filter
(TBPF). Moreover, FIGS. 9A to 9D show resulting images of these
polystyrene and PMMA. As particularly understood from FIG. 8,
detected values (light intensities) of the polystyrene and the PMMA
at a light wavenumber of around 2920 cm.sup.-1 have almost no
difference. However, scanning for a vibration frequency
corresponding to W.sub.R (corresponding to a light wavenumber of
2800 to 3100 cm.sup.-1) provided a clear difference between the
Raman spectra of the polystyrene and the PMMA, which enabled
accurate discrimination and detection of the polystyrene and the
PMMA. This experiment used a titanium sapphire laser having a
repetition frequency of 76 MHz as the light source for the pump
light and used a mode-locked Yb fiber laser having a repetition
frequency of 38 MHz as the light source for the Stokes light.
Embodiment 2
[0053] FIG. 2 shows a configuration of an SRS measurement apparatus
100' that is a second embodiment (Embodiment 2) of the present
invention. Components in this embodiment identical to those in
Embodiment 1 are denoted by the same reference numerals as those in
Embodiment 1, and description thereof is omitted.
[0054] Embodiment 1 described the case where the second pulse light
generator 2 is configured to cause the plural second pulse lights
having the mutually different light frequencies to simultaneously
exit therefrom. On the other hand, this embodiment includes a
second pulse light generator 2' configured to cause the plural
(four as well as in Embodiment 1) second pulse lights having the
mutually different light frequencies .omega..sub.p1,
.omega..sub.p2, .omega..sub.p3 and .omega..sub.p4 to sequentially
exit therefrom. Specifically, this embodiment provides, between the
highly nonlinear fiber 23 and the four PPLNs 24a to 24d, four
optical switches 25a to 25d to sequentially select, of the PPLNs
24a to 24d, one PPLN to which the pulse light from the highly
nonlinear fiber 23 is introduced.
[0055] Moreover, this embodiment provides a light detector 30'
including one photodiode 32 and one lock-in amplifier 33 which are
used to sequentially detect intensity modulation components of the
four second pulse lights (light frequencies of .omega..sub.p1,
.omega..sub.p2, .omega..sub.p3 and .omega..sub.p4) generated by the
stimulated Raman scattering. The calculating unit 34 controls ON
and OFF of the four optical switches 25a to 25d such that the light
frequency .omega..sub.S (from .omega..sub.S1 to .omega..sub.S2) of
the first pulse light is sequentially scanned with respect to the
frequencies of the four second pulse lights. Thus, the calculating
unit 34 acquires a molecular vibration image and a Raman spectrum
in a continuous and wide light frequency range W.sub.R from output
of the one lock-in amplifier 33.
[0056] This embodiment requires a longer time to scan the target
Raman spectrum range as compared with Embodiment 1 that
simultaneously performs the scanning of the light frequency of the
first pulse light for the four second pulse lights. However, in
this embodiment, intensity of the light focusing to the sample 8 at
one scanning is a sum of intensity of the first pulse light and
intensity of one of the four second pulse lights, which is lower
than a sum of the intensity of the first pulse light and
intensities of the four second pulse lights in Embodiment 1.
Therefore, this embodiment can acquire the molecular vibration
image and the Raman spectrum with a lower intensity light as the
light being focused to the sample 8, as compared with Embodiment
1.
Embodiment 3
[0057] Next, description will be made of a case of using, as the
wavelength tunable band-pass filter 12 in the first pulse light
generator 1 described in Embodiments 1 and 2, a wavelength tunable
band-pass filter different from that described in Embodiment 1
(FIG. 3).
[0058] For example, a wavelength tunable band-pass filter 12' shown
in FIG. 4 uses a half mirror 127 whose angle is fixed, in place of
the light deflecting element 122 shown in FIG. 3. The pulse light
emitted from the first light source (YbFL) 11 and transmitted
through the half mirror 127 enters the light dispersion element 125
through the first and second lenses 123 and 124 as collimated
light.
[0059] This wavelength tunable band-pass filter 12' rotates
(swings) the light dispersion element 125 by the driver 129 to scan
the light frequency .omega..sub.S of the first pulse light in the
light frequency range from .omega..sub.S1 to .omega..sub.S2. The
pulse light reflected in the Littrow reflection at the light
dispersion element 125 is again transmitted through the second and
first lenses 124 and 123, reflected by the half mirror 127 and then
collimated by the fiber collimator 126 to exit toward the Yb doped
fiber amplifier 13.
[0060] In this wavelength tunable band-pass filter 12', in
principle, the first and second lenses 123 and 124 may be removed.
However, a small diameter of a light beam (pulse light) entering
the light dispersion element 125 makes a spectral width of the
light progressing to the fiber collimator 126 wide, which may lower
the resolution of the Raman spectrum. Therefore, it is desirable to
provide the lenses 123 and 124.
[0061] Moreover, another wavelength tunable band-pass filter 12''
shown in FIG. 5 may be used. The pulse light emitted from the first
light source (YbFL) 11 enters a light circulator 130 from its
entrance port, exits from an entrance/exit port thereof, is
transmitted through a lens 131 whose focal length is f2 and then
enters the light dispersion element 125 as collimated light. This
wavelength tunable band-pass filter 12'' also rotates (swings) the
light dispersion element 125 by the driver 129 to scan the light
frequency .omega..sub.S of the first pulse light in the light
frequency range from .omega..sub.S1 to .omega..sub.S2. The pulse
light reflected in the Littrow reflection at the light dispersion
element 125 enters the light circulator 130 from the entrance/exit
port and then exits from an exit port thereof toward the Yb doped
fiber amplifier 13.
[0062] As described above, it is only necessary that the wavelength
tunable band-pass filter be configured to change, by changing an
angle of at least one of the light dispersion element that changes
the light frequency of the exiting light according to the incident
angle of the entering light and the introducing optical system that
introduces the light to the light dispersion element, the incident
angle to the light dispersion element. The change of the angle of
at least one of the light dispersion element and the introducing
optical system includes change of the angles of both the light
dispersion element and the introducing optical system.
[0063] The parameters of the pulse light such as the light
frequency, wavelength and repetition frequency described in the
above embodiments are merely examples, and other parameters may be
used.
[0064] Moreover, although the above embodiments described the case
of using the diffractive optical element (diffraction grating) as
the light dispersion element, other elements may be used as long as
they can change the light frequency of the exiting light according
to the incident angle of the entering light.
[0065] In addition, Embodiments 1 and 2 described the case of
producing the plural second pulse lights whose light frequencies
are mutually different by using the light emitted from the one
second light source provided in the second pulse light generator.
However, the second pulse light generator is provided with plural
second light sources emitting lights whose light frequencies are
mutually different. That is, as the second light source, at least
one light source may be provided.
[0066] Furthermore, although Embodiments 1 and 2 described the case
where the second pulse light generator uses harmonic locking
technique, the second pulse light generator is not limited thereto,
and an intensity modulator may be used for the second pulse light
generator.
[0067] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0068] This application claims the benefit of Japanese Patent
Application No. 2011-257894, filed on Nov. 25, 2011, which is
hereby incorporated by reference herein in its entirety.
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